1. Field of the Invention
The present invention relates to an electrophotographic photosensitive member of a laminar structure, and more particularly, to such a photosensitive member of the structure where at least a charge transport layer and a charge generation layer are laminated in succession in this order on a conductive substrate.
2. Description of the Prior Art
Heretofore there are well known electrophotographic photosensitive members using as a photosensitive component an inorganic photoconductive material such as selenium, cadmium, sulfide, zinc oxide and the like.
On the other hand, since it was found that some organic compounds show photoconductive property, many kinds of organic photoconductive materials have been developed. There are known, for example, organic photoconductive polymers such as poly-N-vinylcarbazole, polyvinylanthracene and the like, low molecular weight organic photoconductors such as carbazole, anthracene, pyrazolines, oxadiazoles, hydrazones, polyarylalkanes and the like and organic dyes or pigments such as phthalocyanine pigments, azo pigments, cyanine dyes, polycyclic quinone pigments, perylene type pigments, indigo dyes, thioindigo dyes, squaric acid methine dyes and the like. In particular, since organic pigments or dyes having photoconductivity are synthesized more easily than inorganic materials, and since the bounds of employable compounds showing photoconductivity for an appropriate wavelength range have been enlarged, many kinds of photoconductive organic pigments or dyes have been proposed. As disclosed in, for example, U.S. Pat. No. 4,123,270, No. 4,247,614, No. 4,251,613, No. 4,251,614, No. 4,256,821, No. 4,260,672, No. 4,268,596, No. 4,278,747, No. 4,293,628, etc., there is already known an electrophotographic photosensitive member in which the photosensitive layer is functionally divided into a charge generation layer and a charge transport layer, and in which certain photoconductive disazo pigments are utilized as the charge generation material.
In particular, with respect to azo type pigments, variation of materials is earnestly investigated recently, and some materials are in practical use.
In general, such photosensitive member is provided with a charge generation layer and a charge transport layer superposed in this order on a conductive substrate, in which said charge transport layer contains a strongly electron-donating charge transport material to facilitate the transportation of positive charge, and such photosensitive member is negatively charged.
This is due to a fact that negative charge transporting materials are generally unsatisfactory in performance and are frequency unacceptable for commercial applications due to strong carcinogenecity thereof.
However, negative corona discharge results in formation of significant ozone, giving rise to an additional cost for an ozone filter in the copying machine for removing such ozone. Also periodic maintenance becomes indispensable since such ozone filter deteriorates gradually in performance during prolonged use.
Further the negative corona discharge tends to result in an uneven discharge, for example, due to smears on the discharge wires, thus leading to uneven image density. Also the generated ozone undesirably affects the service life of the organic photoconductor.
Furthermore, the negative corona discharge is associated with deterioration of the surface of photoconductor by the generated ozone and deposition of ionic substances, generated by the discharge, onto the photoconductor, whereby the surface potential thereof is entirely or locally reduced, thus eventually causing entire or local blurs or defects in the electrophotographically reproduced image.
On the other hand, positive corona discharge generates ozone in an amount of 1/5 to 1/10 in comparison with the case of negative corona discharge, is much less associated with uneven discharge caused by the smears of the discharge wires, and is more desirable for the service life of the photosensitive member.
A positively chargeable laminar-structure photosensitive member can be constructed, for example, by superposing a positive charge transport layer and a charge generation layer in this order on a conductive substrate.
However, if such a photosensitive member is used in a copy process, for example, comprising the steps of charging, imagewise exposure, development, transfer of toner images onto a receiving member, separation of the receiving member from the photosensitive member, cleaning and discharging, the surface of a photosensitive member will be gradually abraded off. Such abrasion can occur for example, in the steps of image development, image transfer and cleaning. Consequently, after prolonged used, the sensitivity of the photosensitive member changes to a great extent, or in extreme cases, the photosensitivity will be entirely lost.